The present invention relates to a toner image fixing device for use in an electrophotographic type image forming device such as a copying machine and the like.
Generally, a toner image fixing device for use in an electrophotographic type image forming device is provided with a pair of fixing rollers both of which abut at their cylindrical surface to each other and at least one of which is of a heat roller type incorporating a heater in its body. The heat roller type fixing device is generally such that a toner image developed on a recording material in an image forming step is fixed thereon by heat while the material passes, while being heated, through a nip between the paired fixing rollers, each of which is provided with a separating portion for separating the recording material from the cylindrical surface of the roller, thereby preventing the recording material from adhering to the latter surface.
In this instance, a toner image fixing device for a monochromatic type image forming device uses fixing rollers coated with a Teflon coat having a high strength against mechanical damages, so that the image quality may not be affected by separating portions being always in contact with the cylindrical surface of the rollers.
In contrast, the toner image fixing device for a color type image forming device, in which a color toner image consisting of plural toner layers, being thick and having an uneven surface formed on a recording material, is needed to have fixing rollers having a thick rubber coat capable of fusing and mixing the color toner by sufficiently heating the toner. In other words, the fixing rollers must have a thick rubber layer having a large thermal capacity and tightly covering the uneven surface of the toner image. The thick rubber coated surfaces of the fixing rollers may be damaged if the separating portions are always in contact with the roller surfaces. This results in deterioration of the toner image.
Therefore, a conventional fixing device adopts sheet separating portions capable of being detached from fixing rollers while not forming/fixing a toner image, as disclosed for example in Japanese Laid-Open Patent Publication No. 63-314582, or provides a very small gap between each sheet separating portion and a fixing roller surface.
On the other hand, for a heat roller type fixing device, a fixing roller thermally fixes a toner image developed on a recording material by directly contacting its hot surface with the toner image. Toner particles adhered onto the fixing roller may be undesirably transferred onto a toner image to be fixed on a next recording material.
As such, offset toner is apt to occur in a fixing device of the color type image forming device rather than in a fixing device of the monochromatic type image forming device. This is because the fixing device of the color type image forming device must use fixing rollers with a thick rubber coat having a large heat capacity and covering ability to fuse mixed color toners of a thick and uneven toner developed image consisting of plural toner layers. Such fixing rollers may not possess sufficient release ability due to their thick rubber layer.
For the fixing device of the color type image forming device, the fixing rollers are generally coated with a uniform coat of a release agent (e.g., silicone oil) applied with a coating roller.
By sufficiently applying an anti-offsetting agent to the surface of the fixing rollers, it is realized that a resilient recording material carrying a toner image developed thereon in normal ambient conditions can be easily separated from the fixing rollers even with a sheet separating portion apart from the roller surface.
However, at a humidity considerably higher than a normal humidity level, the recording material may lose its resiliency and not be separated from the fixing roller by the non-contacting sheet separating portions even though the rollers are sufficiently coated with release agent. The recording material may wrap around the fixing roller surfaces. In other words, in the conventional arrangement of the fixing rollers with the separating portions located apart from the roller surface, the recording material in the above condition may not be separated from the separating portions, resulting in jamming it between the fixing rollers.
In the fixing device, where sheet separating portions are used in contact with the fixing roller surfaces while a toner image is formed and fixed on a recording material, oil (anti-offsetting agent) sufficiently applied to the fixing roller may form stripes along the separating portions on the fixing roller surfaces. The oil stripes (contamination with oil) may appear and deteriorate a toner image fixed on a recording material. This contamination is not as visible on usual recording paper but may considerably deteriorate the toner image fixed on a transparent film sheet to be used for an overhead projector. In the latter case, dark stripes appear in the image projected on a screen through the overhead projector since light transmitted through the contaminated portion is reduced by scattering thereat.
An object of the present invention is to provide a toner image fixing device capable of switching sheet separating portions into contacting positions in which the portions are in contact with corresponding roller surfaces or detached positions at which the portions are apart from corresponding roller surfaces in accordance with a condition of the resiliency of a recording material to surely prevent the recording material from wrapping around the fixing rollers and being jammed between the rollers as well as to reduce the frequency of contacting the separating portions with the surfaces of the corresponding rollers, thereby achieving a high quality toner image fixed on the recording material and extending the service life of the fixing rollers.
Another object of the present invention is to provide a toner image fixing device provided with a sheet separating portion for separating a recording material from a cylindrical surface of a fixing roller, the portion capable of changing its position from a contacting position in which it is in contact with the surface of the fixing roller to a detached position in which it is apart from the surface of the fixing roller and vice versa, a sheet separating portion driving means for turning the sheet separating portion into the contacting position or the detached position and a control means for controlling the sheet separating portion driving means based on, at least, either one of the humidity and the kind of the recording material.
Thus, the separating portion can be turned by the sheet separating portion driving means into its contacting position or detached position in accordance with at least one condition regarding the resiliency of the recording material, i.e., the humidity or the kind of the recording material. This allows the separating portion to be used in contact with the fixing roller surface for separating the recording material from the roller surface merely under the condition that the recording material may easily wrap around the fixing roller and may not depart by itself there from.
Furthermore, the frequency of putting the separating portion into contact with the fixing roller surface is restricted to the case when the recording material may easily wrap around the fixing roller, so the frequency of forming an oil stripe on the roller surface by the separating portion can be minimized, preventing possible deterioration of the toner image fixed on the material, and the possibility of damaging the roller surface by the separating portion can also be reduced, thus extending the service life of the fixing roller.
Another object of the present invention is to provide a toner image fixing device for fixing by heating a toner image formed on a recording material passing through a nip between paired fixing rollers, wherein it is provided with paired sheet separating portions, each portion being capable of changing its position from a contacting position in which it is in contact with a cylindrical surface of the corresponding fixing roller for separating the recording material from the roller surface, to a detached position being apart from the surface of the fixing roller and vice versa, a sheet separation portion driving means for turning the sheet separating portion into the contacting position or the detached position, a humidity sensing means for measuring humidity, a recording material identifying means for identifying the kind of the recording material, and a control means for controlling the sheet separating portion driving means based on, at least, either one of the humidity and the kind of the recording material, which are determined by the humidity sensing means and the recording material identifying means, respectively.
Thus, each separating portion can be turned by the sheet separating portion driving means into its contacting position or detached position on the basis of at least one condition regarding the resiliency of the recording material, i.e., the humidity sensed by the humidity sensing means or the recording material kind determined by the recording material identifying means. This allows the separating portion to be used in contact with the fixing roller surface for separating the recording material from the fixing roller surface merely under the condition that the recording material may easily wrap around the fixing roller and may not depart by itself there from.
Furthermore, the frequency of putting the separating portion into contact with the fixing roller surface is restricted to the case when the recording material may easily wrap around the fixing roller, so the frequency of forming an oil stripe on the roller surface by the separating portion can be minimized, thus preventing possible deterioration of a toner image fixed on the material, and the possibility of damaging the roller surface by the separating portion can also be reduced, thereby extending the service life of the fixing roller.
Another object of the present invention is to provide a toner image fixing device wherein a control means controls sheet separating portion driving means to turn corresponding separating portions into contacting positions under a condition that the recording material is identified as thin paper by the material identifying means. In this case, the thin paper being weak in resiliency is surely prevented from wrapping around any fixing rollers regardless of the humidity.
A further object of the present invention is to provide a toner image fixing device wherein a control means controls sheet separating portion driving means to turn corresponding separating portions into detached positions under the condition that a recording material is identified as thick paper (i.e., thick paper being thicker than normal recording paper and not easily wrapping around the surface of a fixing roller) by a recording material identifying means and a humidity lower than a normal humidity is detected (i.e., at which the paper may not wrap around the fixing roller), by a humidity sensing means. In this case, the frequency of contacting the sheet separating portions with the surfaces of the corresponding fixing rollers and the possibility of damaging their surfaces by the separating portion is minimized, thereby assuring extended service life of the fixing rollers.
A still further object of the present invention is to provide a toner image fixing device wherein a control means controls sheet separating portion driving means to turn separating portions into contacting positions under the condition that a recording material is identified as normal recording paper by a recording material identifying means and a humidity higher than a normal humidity is detected by a humidity sensing means. In this case, since the recording paper of a normal thickness may reduce its strength (resiliency) at a high humidity and therefore may easily wrap around one of the fixing rollers, the sheet separating portions are used in contact with the surfaces of the fixing rollers to surely separate the paper from the roller surfaces. In contrast, the sheet separating portions are kept apart from the fixing rollers at a normal humidity. This reduces the frequency of contacting the sheet separating portions with the fixing roller surfaces and therefore minimizes the possibility of damaging the surfaces of the fixing rollers by the sheet separating portions, thereby extending the service life of the fixing rollers.
Another object of the present invention is to provide a toner image fixing device wherein a control means is capable of controlling selectively the sheet separating portion driving means to turn each separating portion into a contacting or detached position under the condition that a recording material is identified as thick paper thicker than a normal recording paper by a recording material identifying means and a humidity higher than a normal humidity is detected by a humidity sensing means. In this case, since the thick recording paper has a high strength (resiliency) in a normal state but may reduce its strength at a high humidity and therefore may wrap around one of the fixing rollers, the selection is left for a user to turn the sheet separating portions into their contacting positions or detached positions by the user""s preference (for example, the detached position is selected to keep the quality of the fixed toner image and contacting position is selected to eliminate the possibility of jamming of the fixing process at the sacrifice of the quality of the fixed toner image). In practice, it is left for the user to select when to put the portion into the contacting position or to keep the portion in the detached position under the condition that the paper is thick and the humidity is high. This feature may satisfy the user""s demand.
A further object of the present invention is to provide a toner image fixing device wherein a control means controls sheet separating portion driving means to turn separating portions into detached positions under the condition that a recording material identifying means identifies a recording material as a transparent film sheet to be used in an overhead projector. In this case, since the transparent film sheet may be less affected by humidity and has a high strength in comparison with recording paper, there is no fear of breaking the fixing process with the sheet jammed between fixing rollers without using the sheet separating portions.
This eliminates the possibility of deteriorating the fixed toner image with stripes of contamination oil formed by the separating portions on the fixing rollers.